To synthesize 4-chloro-5-methoxy-2-(beta-trifluoroethyl)-3(2H)-pyridazinone (4), the reactions of 4-chloro-5-methoxy-3(2H)-pyridazinone (5) with RCH(2)CF(3) (R = I, TsO, MsO, TfO) in different solvents were studied. It was found that methyl group migration took place during this reaction. An oxonium salt 9 was suggested as the active intermediate for the formation of the byproduct 4-chloro-5-methoxy-2- methyl-3(2H)-pyridazinone (7) and 4-chloro-2-methyl-5-(beta-trifluoroethoxy)-3(2)-pyridazinone (8).